This invention relates to capacitive proximity instruments and particularly to instruments having both circuit boards and capacitive electrodes within closed dielectric housings.
Proximity type of instruments are adapted to be installed in containers for providing control or warning signals in response to material closely approaching or surrounding sensing elements of the instruments. By placing different sensing elements at different predetermined levels in containers or bins, signals for indication or control are provided in response to materials within the containers having reached the respective levels.
Certain prior instruments are used namely for granular materials. For example, one type has paddles placed in grain bins. When the paddles are above the level of grain, they are rotated freely by low-torque motors, but when the grain reaches the level of each paddle in succession, the paddles are stopped by the grain restraining their rotation. Signal means are operated in response to the stopping of motors to which the paddles are connected.
A prior capacitive type of level measurement equipments have probes that are single electrodes extending a short distance inwardly from mounting positions in the walls of bins. Adjacent each probe, the lines of force of an electric field extend outwardly from the probe and bend toward a conductive mounting base or to the adjacent wall of the bin in which the probe is placed. As a material having substantially greater dielectric constant than air comes with the field, the capacitance of the probe increases sufficiently to cause an output from the circuits to which it is connected. Since the effective field is spread over quite a large space about a probe of this type, and since the probe may need to be substantially covered by material before obtaining output indication, sufficient sensitivity is not obtained to detect precisely when a predetermined level has been reached. For example, A. Edelman et al in U.S. Pat. No. 2,774,959 issued on Dec. 18, 1956 teach the use of a single rugged probe suitable for sensing the level of heavy agglomerate materials.